WASHINGTON — After pulling all remaining American diplomats from Venezuela on Thursday, the Trump administration warned the country’s leader, Nicolás Maduro, that any pressure on Juan Guaidó, the self-declared interim president who has the White House’s support, would be met with an “immediate reaction” from the international community.

But it remained unclear what that would mean. The administration issued the threat in the midst of a diplomatic retreat that has cast doubt on the United States’ ability to reinforce Mr. Guaidó if Mr. Maduro’s government moved to arrest him on accusations of orchestrating a dayslong blackout that plunged Venezuela deeper into unrest before power was restored in most major cities on Tuesday.

“We hold former President Maduro and those surrounding him fully responsible for the safety and welfare of interim president Juan Guaidó and his family,” Robert Palladino, a State Department spokesman, said on Thursday. “It would be a terrible mistake for the illegitimate Maduro regime to arrest Juan Guaidó.”

Mr. Palladino said the United States would be seeking to recruit a country to act as a protecting power and support any United States citizens still in Venezuela. But in a briefing with reporters, Mr. Palladino urged Americans to “strongly consider” departing the country.

As conditions in Venezuela continue to deteriorate, the Trump administration has stuck to a sanctions-based policy of trying to pressure Mr. Maduro, who has shown little sign that he will acquiesce to the request of the United States, nor over 50 other countries that back Mr. Guaidó.

Human rights groups believe Mr. Maduro’s government has detained dozens of people accused of helping orchestrate the blackout. The arrest of Luis Carlos Díaz, a prominent radio producer and social media activist, has rattled people who worry that Mr. Guaidó could be next.

Mr. Palladino said on Thursday that the Trump administration had revoked over 600 visas of Venezuelans, including those belonging to more than 100 people the administration says were diplomats or otherwise close to Mr. Maduro.

“Policy is going to continue to support democracy in Venezuela,” Mr. Palladino said, without bringing up the possibility of military intervention, a tactic Trump administration officials have previously identified as an option, albeit a distant one.

The administration, Mr. Palladino said, would continue “using sanctions and diplomatic pressure to pressure the illegitimate regime to end.”